This application is related to Japanese patent applications No. Hei 11(1999)-195388 filed on Jul. 9, 1999 and No. Hei 11(1999)-090528 filed on Mar. 31, 1999, whose priorities are claimed under 35 USC xc2xa7119, the disclosure of which is incorporated herein by reference in its entirety.
The present invention relates to an optical device capable of making it compact and a method for producing the same, and to a method for producing a polyimide film.
With the practice in the variety of mobile electronic appliances and network equipments, further development for a more compact and lightweight optical device having a higher performance is required. Main types of conventionally known optical devices include, firstly, an optical waveguide formed on a substrate as an optical device for communication or the like, and secondly, a bulk-type optical device comprising a combination of various types of optical components.
The optical waveguide referred above requires that the light loss is small, that it is easily produced, that the difference in refraction index between the core layer and the clad layer is controllable, that it has excellent heat resistance, etc. However, since a quartz based optical waveguide require a process at a temperature as high as 1,000xc2x0 C. or even higher, there are such problems that the substrate on which the optical waveguide is formed is limited, and that the waveguide cannot be formed after the optical components are mounted on the substrate.
In the case of optical communication systems using a plastic optical fiber (POF) as the transmission medium, which is recently attracting attention, a thick film optical waveguide capable of being efficiently connected with a large POF having a diameter of about 1 mm is necessary. However, it is difficult to obtain thick films of a quartz based optical waveguide due to the problems encountered in their production.
On the other hand, many are interested in the recently proposed plastic optical waveguide, because it enables waveguides at lower temperatures, and thick films thereof are relatively easily achieved. A polyimide optical waveguide using polyimide, which boasts the highest heat resistance among the plastics, can be generally produced by a method as follows:
(1) producing a thin film of polyimide by coating a substrate with a solution of polyamic acid or polyimide by means of, for instance, spin coating, and then heating it remove the solvent by volatilization; and
(2) obtaining a polyimide optical waveguide by patterning the resulting thin film into a desired shape by means of, for example, dry etching using gaseous oxygen.
In the case of an optical device using such as POF as the transmission medium, the loss increases in the wavelength region of from about 600 nm to 800 nm. Thus, in a practical application, it is necessary to minimize the loss. Accordingly, in JP-A-Hei4-9807 (the term xe2x80x9cJP-A-xe2x80x9d as referred herein signifies xe2x80x9can unexamined published patent applicationxe2x80x9d) is proposed a polyimide optical waveguide having a lower loss. The proposed method provides a polyimide having reduced loss by fluorinating the diamine which constitutes the polyimide.
However, the production process for the plastic optical waveguide above had disadvantages that the resulting polyimide optical waveguide exhibits a tanned color characteristic of polyimide; an optical waveguide having low loss was thereby unfeasible.
A fluorinated polyimide disclosed in JP-A-Hei4-9807 was so expensive that an optical device using such a material also suffered a disadvantage of increased cost. Furthermore, in case of processing the polyimide by means of dry etching using gaseous oxygen, the polyimide was found unsuitable for use as a mask, because the selective ratio between the polyimide and a positive resist using novolak based resin was so small, and this inevitably required the use of, for example, silicon oxide, aluminum, etc. This also led to a problematic increase in production cost due to the need of a complicated production process. Although there is known a method of simplifying the production process comprising using a polyimide having a photosensitive group (a photosensitive polyimide), the application of such photosensitive polyimides to optical devices is not studied heretofore because such polyimides undergo coloring upon baking.
As described above, in a plastic optical waveguide heretofore using a polyimide or a like organic polymer, which is relatively easy to obtain thick films therefrom, it has been found difficult to achieve both high performance and low cost at the same time due to the absorption of the core material or to the large optical loss caused by the scattering at the interface between the core and the cladding. Furthermore, it has been found difficult to obtain the final products at a low cost due to the complicated production process.
In the light of the aforementioned circumstances, the present invention has been accomplished in order to solve the problems above. Accordingly, an object of the present invention is to provide a low cost optical device which can be easily made compact and lightweight, and to provide a method for producing the same.
The present inventors have found that a polyimide film having high transmittance can be achieved by forming a coating of a polyimide or photosensitive polyimide on a substrate, and by then baking it in vacuum. This allows a low cost optical device yet having high performance by etching the film into a desired shape.
Thus, in accordance with an aspect the present invention, there is provided a method for producing an optical device having an organic polymer film through which a light beam is transmitted, which comprises applying a solution containing an organic polymer film-forming starting material on a substrate to form the applied film and then baking the applied film under vacuum to form the organic polymer film.